Replication-defective type-C sarcoma viruses arose as a result of recombination between replication-competent type-C retroviruses and host cellular sequences. In the Moloney murine sarcoma virus system (M-MSV) the incorporated cellular sequences have been shown to be involved in the sarcoma virus mediated transformation of fibroblasts in vitro. Current research is designed to identify and isolate the M-MSV coded messenger RNA(s) which are responsible for the process of cellular transformation. Messenger RNAs from nonproducer transformed cell lines are purified by molecular hybridization to specific fragments of M-MSV plasmid DNA bound to nitrocellulose filters. Sarcoma-specific RNA is eluted and translated in a reticulocyte cell-free system to identify the protein(s) product(s) which may be involved in fibroblast transformation. Sera from animals bearing M-MSV-induced tumors are tested for their ability to immunoprecipitate sarcoma coded proteins.